The great tower of the Old Man of Hoy has vertical or overhanging walls. The stack is separated from the adjacent cliffs by a 60m wide chasm. The base of the pinnacle, like the adjacent cliffs of St John’s Head, rests upon a pedestal of dark basalt lava above which horizontal beds of red and yellow sandstones rise. The alternating beds of relatively soft, sandy and pebbly sandstone with occasional beds of harder grey flagstone, give the Old Man a blocky, notched and often overhung profile. The base of the Old Man rises from a shore platform which is littered with blocks derived from collapse of a former arch.
The Old Man is a geological juvenile, probably less 400 years old and perhaps soon to collapse (Evans and Hansom, 1995). There is no mention of this striking feature in the Orkneyinga sagas. On the Bleau map of around 1600, the Old Man is not represented and presumably not yet formed and a headland exists with the fort of Braburgh on the point. Similarly, the McKenzie map of Hoy in 1750 shows a headland but no stack. By 1819, when the landscape painter William Daniell visited Hoy and sketched the site, the headland had been eroded into a stack and arch with the twin legs that gave The Old Man its name (Daniell, 1821). Early in the 19th century, a severe storm washed away one of the legs (Miller, 1976). Erosion continues today and by 1992, a 40m crack had opened up in the top of the south face to leave a large overhanging block that will eventually collapse.